terminology on time/pacing

Can someone clarify for me what is meant when we refer to someone as a say 1:40 per 100 m swimmer? Does this refer to their sustainable/holdable pace per 100 m, or does this mean their best/fastest pace per 100 m?

This is the sustainable pace you'd be able to do a 1500m at. At least, that's my understanding of Swim Smooth's Critical Swim Speed. This is what I coach my triathletes when we time them on a 400 and 200, and then do the math on the iPod. I tell them their 100m pace, and tell them this is the pace they should be able to maintain for a 1500m swim in the LCM pool.

Then, if they want to do an IM swim (4K-ish), then I tell them to add :05 to :10 per 100m, for example.